Hair dyes can be classified by the dye to be used or by whether they have any bleaching action on melanin or not. Typical examples include a two-part permanent hair dye composed of a first part containing an alkali agent, an oxidation dye and optionally a direct dye such as a nitro dye and a second part containing an oxidizing agent; and a one-part semi-permanent hair dye containing an organic acid or an alkali agent, and at least one direct dye such as an acid dye, basic dye or nitro dye.
The above-described two-part permanent hair dye however has drawbacks that the color tone imparted by the oxidation dye is not so vivid or that the color of the hair dyed with the nitro dye, which is ordinarily employed as a direct dye and produces a vivid-color, markedly fades over time and becomes dull quickly although the color tone immediately after dyeing is very vivid. A variety of direct dyes such as cationic direct dyes and nitro dyes have been used in combination in permanent hair dye products in order to produce a vivid color.
Direct dyes available at present however cannot bring about sufficient effects. Moreover, the number of direct dyes that can be used in combination with an oxidation dye is limited because they are required to have stability against alkali peroxides during the hair dying process. In any case, the color fades very quickly due to the loss of the direct dye as a result of washing or exposure to light. This phenomenon is marked in damaged or porous hair (hair having pores formed therein).
The present inventors provided azo dyes having a dissociative proton as a direct dye capable of overcoming the above-described problems (for example, refer to Patent Documents 1 and 2). These dyes however still have problems to be improved, such as resistance to light, shampooing, sweat, friction or heat, or stability against an alkalizing agent or an oxidizing agent.
[Patent Document 1] JP-A-2003-342139
[Patent Document 2] JP-A-2004-107343